Many of the original vegetarian and vegan diets in Asia were also helped with sadhanas to raise the pranic lifeforce. When the pranic energetic force is raised one can live on very small amounts of foods and not have any loss of energy.
If people who had former meat diet then try to change rapidly it may have some early bad effects on health, but on if they are buddhist can overcome this by raising the pranic energy by practice. Fats are addictive, so the body will crave initially, but this can be overcome with practice.
I know many vegans vegetarians that were former meat eaters, I was myself about 30 years ago and have enjoyed great health and sustained good practice, by actually putting these things as a whole into practice. If one energy level is low vibration it maybe hard to give up any form of craving for the flesh, this does not mean there is a problem with the yogic ways, some want to make excuses that in Asia some ate meat, usually this was in severe conditions and environments where food was hard to come by and would at least limit the amount they took.
Do some detox, fasting is good, half a day a week is good, clean the system, retrain the body, it may take some practice, but if one has compassion its worth it, killing animals for food in this day and age is not neccessary, people are obsessed with the body, practice take you out of the bodily concept and into more free states of being, not depending on phenomenon or using phenomenon is the most limited ways. This is real liberation, not being a slave to desires and demands of the Body and mind.
There are still many yogis in India that eat almost nothing, and have great energy, some can be enlightened and eat also, it becomes a choice, often times to set a standard.
Be kind to animals, they dont like being killed, they feel fear.